A suitable device is needed for the direct measurement of oxygen partial pressure (PO2) in blood and tissue for both clinical and research applications. Methods currently available for measuring PO2 lack convenience, reliability, speed, and relevance to many situations of interest. Efforts to develop electrical sensors have not been successful. It is desirable to have a very small PO2 sensor which can be inserted into a blood vessel or tissue with little disturbance, and which will provide instantaneous and current PO2 monitoring for either short or extended periods of time. A fiber optic sensor is ideal for this application, with the advantages, for physiological use, of very small size and flexibility, safety, and low cost. A PO2 probe was developed based on the principle of fluorescence quenching by oxygen. It consists of two 150 micrometer strands of plastic optical fiber ending in a section of porous polymer tubing about 3 mm. long and 0.6 mm. diameter. The tubing is packed with dye on an adsorptive support. The optical fibers are connected to a light source and measuring instrument which provide direct PO2 reading. The general construction is similar to a pH probe previously developed here. Three major problems had to be solved: finding a suitable fluorescence indicator, devising an oxygen permeable container, and developing a support system for the dye which would activate it without humidity sensitivity. Appropriate instrumentation was developed for use and evaluation of the probe. The probe has been evaluated with in vitro and animal tests. There is considerable interest in application of the PO2 probe, and instrumentation development for specific research projects is planned.